Composition for photo imaging

ABSTRACT

An improved photoimagable cationically polymerizable epoxy based coating material is provided. The material includes an epoxy resin system consisting essentially of between about 10% and about 80% by weight of a polyol resin which is a condensation product of epichlorohydrin and bisphenol A having a molecular weight of between about 40,000 and 130,000; between about 20% and about 90% by weight of an epoxidized octafunctional bisphenol A formaldehyde novolak resin having a molecular weight of 4,000 to 10,000; and if flame retardancy is required between about 35% and 50% by weight of an epoxidized glycidyl ether of tetrabromo bisphenol A having a softening point of between about 60° C. and about 110° C. and a molecular weight of between about 600 and 2,500. To this resin system is added about 0.1 to about 15 parts by weight per 100 parts of resin of a cationic photoinitiator capable of initiating polymerization of said epoxidized resin system upon exposure to actinic radiation; the system being further characterized by having an absorbance of light in the 330 to 700 nm region of less than 0.1 for a 2.0 mil thick film. Optionally a photosensitizer such as perylene and its derivatives or anthracene and its derivatives may be added.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to photoimagable compositions, and moreparticularly to a cationically polymerizable epoxy resin system havingphotoinitiators and optionally photosensitizers added thereto whichresin system has improved rheological properties and also improvedphotoimaging properties.

2. Background of the Prior Art

There are many different instances where photoimagable compositions areutilized in various industrial processes. In one particular process aphotoimageable composition is utilized as a solder mask by applying thecomposition to the underlying printed circuit board. Thereafterphotolithographic techniques are employed to reveal various underlyingstructures on the board while masking others so that solder may beapplied by various solder applying processes to the exposed structures.During the solder applying process the solder will adhere to the exposedunderlying components and be prevented from adhering where the remainingmaterial operates as a solder mask.

It is necessary that the solder mask material be formulated such that itcan be applied by the appropriate methods; for example curtain coating,a preferred method, requires certain rheological properties foreffective coating. Further, the solder mask must have the properties ofproviding efficient transmission of the light or other exposingradiation so as to photolyze the photoinitiator through whateverthickness of material is applied. Also, of course, if the material is tobe used as a solder mask, the solder mask must possess appropriatephysical and chemical properties to withstand the application of thesolder material without significant deterioration or degradation andmaintain its coverage over the portions of the board wherein solder isto be masked. If it is to be used for other purposes, other propertiesmay be required.

There have been many prior art proposals for different photoimageablecompositions including many that use epoxies. Examples of these arefound in the following U.S. Pat. Nos.: 4,279,985; 4,548,890; 4,351,708;4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623;4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197.All of these patents show various resins and photoinitiators for use inphotoimageable compositions many of which are useful as solder masks.However none of them teach or suggest the specific composition of thepresent invention.

SUMMARY OF THE INVENTION

According to the present invention, an improved photoimagablecationically polymerizable epoxy based coating material is provided. Thematerial includes an epoxy resin system consisting essentially ofbetween about 10% and about 80% by weight of a polyol resin which is acondensation product of epichlorohydrin and bisphenol A having amolecular weight of between about 40,000 and 130,000; between about 20%and about 90% by weight of an epoxidized octafunctional bisphenol Aformaldehyde novolak resin having a molecular weight of 4,000 to 10,000;and between about 35% and 50% by weight of an epoxidized glycidyl etherof tetrabromo bisphenol A having a melting point of between about 90° C.and about 110° C. and a molecular weight of between about 600 and 2,500if flame retardant properties are desired. To this resin system is addedabout 0.1 to about 15 parts by weight per 100 parts of resin of acationic photoinitiator capable of initiating polymerization of saidepoxidized resin system upon exposure to actinic radiation; optionally aphotosensitizer in an amount of up to about 10 parts by weight may beadded to enable/enhance exposure at other wave lengths; the resin systemexclusive of the photoinitiator and sensitizer being furthercharacterized by having an absorbance of light in the 330 to 700 nmregion of less than 0.1 for a 2.0 mil thick film.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a photoimageable coating material whichincorporates a cationically polymerizable epoxy resin system having aphotoinitiator and optionally a photosensitizer therein. The system wasspecifically developed for application by curtain coating techniques andfor use as a solder mask on a printed circuit substrate. However, thesystem can be used for other applications, such as an etch mask, andalso can be coated by other techniques such as roller coating. In thisparticular solder mask application, the photoimageable material istypically curtain coated by conventional curtain coating techniques ontoa substrate to a thickness of about 0.5 to about 4.0 mils or more,dried, photoimaged and developed. The developed areas reveal the desiredunderlying metallized portions of the substrate wherein solder is to beapplied, and the remaining solder mask material is cured and remains onthe board as a solder mask during the application of solder by anyconventional technique. Thus the system of this invention must havesatisfactory rheological properties for application by curtain coatingtechniques, it must be sensitive to exposure to radiation, whichconventionally is in the 330 to 700 nm region, without any significantabsorption to thereby allow penetration of the exposure radiationcompletely through the film; and it must also have the necessaryphysical and chemical properties to resist degradation during thesoldering process. Conventionally the solder mask remains on the boardafter soldering and hence for many applications must be fire or flameretardant. In formulating such a coating there are many competingconsiderations that must be considered in order to provide a desired endproduct. No specific type of epoxy resin has been found which willsatisfy all of the various requirements; however, a formulation of acombination or mixture of various epoxy resins according to thisinvention provides the required properties for a curtain coatablephotoimageable flame retardant solder mask composition.

In general, the epoxy resin system consists essentially of a phenoxypolyol resin of a high molecular weight which is a condensation productbetween epichlorohydrin and bisphenol A. A suitable resin of this typeis sold by Union Carbide Corporation under the Trade Mark PKHC. Thisresin has an epoxide value of about 0.03 equivalents per kg, a weightper epoxide of about 37,000, and a Tg (glass transition temperature) ofabout 98° C. The second resin in the system is an epoxidizedoctafunctional bisphenol A formaldehyde novolak resin with a mediumrange molecular weight. A suitable resin of this type is sold by HighTek Polymers under the Trade Mark EpiRez SU-8. This resin has an epoxidevalue of about 4.7 equivalents per kg, a weight per epoxide of about 215and a melting point of about 82° C. These two resins will provide thenecessary rheological, photolithographic and physical propertiesnecessary to curtain coat for solder mask applications. However, in manyinstances, flame retardancy is desirable, and these two resins do notexhibit adequate flame retardancy for many applications. In such cases athird resin is added for flame retardancy. The third resin in the systemis a low molecular weight high softening point epoxidized glycidyl etherof tetrabromo bisphenol A. A suitable resin of this type is also sold byHigh Tek Polymers Corporation under the Trade Mark EpiRez 5183. Thisresin has an epoxide value of about 1.5 equivalents per kg, a weight perepoxide of about 675, and a melting point of about 97° C. Aphotoinitiator such as various sulfonium salts, iodonium salts, andferrocene salts is added to the resin system for providing the properphotoresponse to actinic radiation. Since the resin system iscationically photocurable, the photoinitiator must be capable of causingcationic polymerization of the resin upon exposure to radiation. Oneparticularly desirable photoinitiator is a complex triarylsulfoniumhexafluoroantimonate salt sold by General Electric Company under theTrade Mark UVE 1014. Other photoinitiators such triphenylsulfoniumhexafluorophosphate sold by General Electric Co. under the Trade MarkUVE 1016, and diphenyliodonium hexafluoroantimate may be used.Optionally a photosensitizer such as anthracene or its derivatives orperylene or its derivatives can also be added which may increase theresponse speed of photoimaging and/or the wavelenth sensitivity. Thedifferent epoxy resins described above, selected to be within certainmolecular weight ranges, are blended in certain percentages.

It has been found generally that from about 10% to 80% of the polyolresin is preferred with the molecular weight being generally in therange of 40,000 to 130,000, and 20% to 90% of the epoxidized novolakresin is preferred with a molecular weight of between about 4,000 and10,000. This is the most general case in which flame retardancy is not acriterion. However, flame retardancy is quite often a desirable if notrequired property. In such cases, it has been found that generally about20% to 40% and preferably 25% to 35% of the polyol resin can be usedwith the molecular weight being generally in the range of 40,000 to130,000 and more specifically and preferably in the range of 60,000 to90,000. About 25% to 35% and preferably 25% to 30% of the epoxidizednovolak resin can be used with a molecular weight generally of about4,000 to 10,000 and preferably from about 5,000 to 7,000. About 35% to50% epoxidized brominated bisphenol A is desirable and more preferablyabout 40% to 45% with a molecular weight of generally about 600 to 2,500and preferably about 1,000 to 1,700 can be used. The photoinitiator, andoptionally if desired the photosensitizer is added to the resin system.From about 0.1 and about 15 parts by weight of the photoinitiator basedon 100 parts of the resin system are normally what is required andoptionally up to about 10 parts by weight of a photosensitizer based on100 parts of the resin system can be used. (It is conventional practicein the art of photoimaging to designate the percentage of the componentsof the resin system to add up to 100% and to designate the additions oradditives thereto in values of parts by weight based on 100 parts of theresin system, and this convention is adopted herein.)

The specific composition is selected to optimize the desired properties.For example, the PKHC controls the rheology of the coated materials, the5183 imparts flame retardant properties to the material, and the SU-8imparts fast photo speed and improved resolution. In selecting thespecific amount of each resin it will of course be understood that byincreasing the concentration of any one of the resins, this would alsoincrease the respective property associated with it; however, increasingthe concentration of any one of the specific resins would require adecrease in the concentration of one or both of the remaining resinswhich would result in a decrease of the function of the specificproperties associated therewith. A decrease in any of the resins belowthe broad range percentages indicated above would result in propertieswhich are unacceptable for the specific purpose of curtain coatableflame retardant high resolution photoimageable solder mask material.

As the PKHC amount is decreased, the resulting rheology results in areduced coverage when the material is applied and the resulting materialis extremely brittle. By reducing the 5183, the resulting system hasdecreased flame retardant characteristics and, if present below theminimum specified broad range, the flame retardant qualities would notmeet certain specified industry requirements, e.g. UL 94 V0 FlammabilityRequirements. However, any amount of the 5183 does provide some flameretardant characteristics. When the SU-8 concentration is reduced, aslower photospeed and lower resolution results to the extent that if itis decreased below the minimal level as given above, the photo speed isunacceptable for this particular application. Thus in balancing theformulation of the material, all of these requirements must beconsidered and the end product optimized to give satisfactory propertiesfor the result desired.

The following four formulations were found to perform very well assolder mask compositions:

                                      TABLE I                                     __________________________________________________________________________                   UVE                                                                    EpiRez 1014                                                               PKHC                                                                              5183                                                                              SU-8                                                                             PARTS/                                                                             Tg(a)                                                                              Tg(b)                                                                             Log G'(a)(c)                                     Exam.                                                                             %   %   %  WT.  Coating                                                                            Coated                                                                            @ 100° C.                                 __________________________________________________________________________    1   20.0                                                                              50.0                                                                              30.0                                                                             5    73       5.5                                              2   27.3                                                                              45.4                                                                              27.3                                                                             5    75       5.9                                              3   33.3                                                                              41.7                                                                              25.5                                                                             5    78       6.1                                              4   30.0                                                                              45.0                                                                              25.0                                                                             5    78   125 6.4                                              __________________________________________________________________________     (a) Determined by a Rheometrics Dynamic Spectrometer, Model 7700 with 1"      parallel plates.                                                              (b) Determined using a PerkinElmer DSC7.                                      (c) This is the log of the storage modulus @ 100° C.              

Each example was mixed in a propylene glycol monomethyl ether acetate(PGMEA) solvent, these formulations being about 40% solids in the PGMEA.Each of these particular formulations in this solvent coated very wellonto a circuit board. However, other moderately polar solvents can beused such as propylene glycol monomethyl ether, 2-methoxyethanol,2-methoxyethyl acetate, 2-ethxyethyl acetate, N-methyl pyrrolidone,propylene carbonate, or gamma-butyrolactone.

The preferred formulation for a solder mask is in Example 4.

The following curtain coating techniques were used involving theapplication of the photoimageable material to a substrate via a freefalling "curtain" of material. As the substrate passes through thecurtain, it is coated with the material. The curtain coating set-upincludes a pan, pump, viscosity controller, and a coating head. Thematerial to be coated is pumped from the pan to the coating head, wherethe curtain is formed. This curtain falls back down into the pan,recirculating constantly, with exception of the material deposited ontosubstrates as they pass through the curtain. The coating viscositiesrange from 200-2,000 Cps, and typica dried film thicknesses deposited onthe substrate range from 0.5.-4.0 mils.

Other types of coating, such as roll coating and wound wire rod coating,can also be used.

These particular formulations of epoxy resins and photoinitiators coatedextremely well by the curtain coating methods without any spaces or gapsand covered the circuit board to a thickness of about 2 mils. During thephotoimaging process, the light penetrated essentially completelythrough the coating to the underlying structure; thus the coatingexclusive of the photoinitiator and sensitizer was transparent to thisparticular light, i.e. had an absorbance of less than 0.1 for the 2.0mil thick film.

The solder mask material was exposed to UV radiation from a mediumpressure mercury lamp. The UV radiation was passed through a phototoolwhich is opaque in those area where the solder mask is to be removed.After exposure to UV radiation, the circuit boards were baked for ashort time to accelerate the crosslinking reaction initiated by thesulfonium salt photolysis products. Bake temperatures between 100° C.and 150° C. and times between 2 and 10 minutes were used. The soldermask was then developed using a solvent that easily dissolves anyunexposed material. The preferred developer is DY90 sold by Ciba-Geigywhich contains gamma Butyrolactone, Propylene Carbonate, and Diglyme;however, Butyrolactone alone works well as a developer. Spray developingequipment and other physical agitation of developer solution was used.Curing of the solder mask was accomplished by exposing the solder maskto UV radiation followed by an oven bake at 150° C. UV doses varied from1 to 4 Joules/sq cm. Bake times varied from 30 to 90 minutes. The Tgvalues as applied for all four examples and as cured for Example 4 areshown in Table 1.

After removing surface oxides such as with a dilute HCl rinse andapplying an antioxidant such as benzotriazole, circuit boards can besoldered using a variety of methods. Two of the processes used withsolder mask described here are wave solder and vapor phase solder.

Wave solder involves placing components onto the circuit board, coatingthe metallic surfaces to be soldered with a flux and passing thisassembly through a continuously flowing wave of molten solder. Vaporphase solder is accomplished by applying flux and solid solder to theexposed metallic surfaces on the board by screening or some otherappropriate technique. After placing components on the board theassembly is passed through a vapor which is maintained at a temperatureabove the melting point of the solder.

Sample 4 was tested for solderability in a conventional manner using thefollowing "solder shocking" test which is a common method of testingsolder mask performance under typical soldering stresses seen inindustrial processes.

The coated substrate is photoimaged, developed, and fully cured. Thepart is submersed in molten solder, maintained at 500° F.±25°, for 10-20seconds. After the submersion into the solder, the part is allowed tocool to room temperature and inspected. Visual inspection for cracking,flaking, blistering, or noticeable degradation of the material isperformed. If the solder shock does not physically degrade the material,as confirmed by visual inspection, then the solder mask passes thesolder shock test criteria.

For some applications, certain additives to the formulation may bedesired. For example, a fluorescent or color dye may be added forinspection or cosmetic purposes. These would be present normally inquantities of about 0.001 to about 1 part per 100 by weight. Examples ofthese types that have been used are malachite green oxalate, ethylviolet, and rhodamine B. Additionally for some coating applications, itmay be desirable to use a surfactant, e.g. Fluorad FC 430 sold by 3MCorp. The Surfactant will normally be present in quantities of 0.01 to 1part per 100 by weight. It will, of course, be understood that theadditive must not significantly degrade the other properties of thecoating. Table II below shows several examples of the preferredembodiment with various additives.

                  TABLE II                                                        ______________________________________                                                                    UVE.sup.b                                         Exam. PKHC.sup.a                                                                             5183.sup.a                                                                           SU-8.sup.a                                                                          1014  Surfactant.sup.bc                                                                     DYE.sup.b                           ______________________________________                                        5     30       45     25    5     0.03    --                                  6     30       45     25    5             0.05*                               7     30       45     25    5     0.03    0.05*                               8     30       45     25    5     --      0.05**                              9     30       45     25    5     0.03    0.05**                              ______________________________________                                         NOTES:                                                                        .sup.a Percentage by weight.                                                  .sup.b Parts per hundred parts resin, by weight.                              .sup.c Fluorad FC 430                                                         *Malachite Green                                                              **Ethyl Violet                                                           

Table III shows three examples where just PKHC and SU-8 are utilized.These are useful where flame retardancy is not required.

                  TABLE III                                                       ______________________________________                                                                       Log G'                                         Exam.   PKHC(a)       SU-8(a)  @ 100° C.                               ______________________________________                                        10      50            50       6.3                                            11      73            27       6.7                                            12      25            75       4.3                                            ______________________________________                                         (a) Percentage by weight.                                                

Examples 10 and 11 have good characteristics for curtain coating and useas a solder mask. While example 12 has generally good properties it hasa rather low Log G' at 100° C. With a value this low, it is notparticularly useful in solder mask applications since it has reducedconformal properties, but it will find use in other applications.

Various other types of resin formulations were found to beunsatisfactory either rheologically and/or photolithographically.

This invention has been described specifically as a formulation forcurtain coating solder mask material. However, it can be used for otherpurposes and may be applied by various other techniques. Also, othertypes of coating, such as roller coating can be used to apply thematerial. Where different uses and coating techniques are employed,certain modifications of the preferred material may be necessary to meetdifferent requirements. For example, a different solvent may be desiredfor roller coating as opposed to curtain coating and the rheologicalproperties may be varied from the optimum for curtain coating for thedifferent types of uses.

While the invention has been described with a certain degree ofparticularity, various adaptations and modifications can be made withoutdeparting from the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. A photosensitive cationically polymerizable epoxybase imaging system comprising;an epoxy resin system consistingessentially of between about 20% and about 40% by weight of a polyolresin which is a condensation product of epichlorohydrin and bisphenol Ahaving a molecular weight of between about 40,000 and 130,000; betweenabout 20% and about 35% by weight of a epoxidized octafunctionalbisphenol A formaldehyde novolak resin having a molecular weight ofbetween about 4,000 and about 10,000; between about 35% and about 50% byweight of an epoxidized glycidyl ether of tetrabromo bisphenol A havinga softening point of between about 60° C. and about 110° C. and amolecular weight of between about 600 and about 2,500; and from about0.1 to about 15 parts by weight of a cationic photoinitiator capable toinitiating polymerization in said epoxidized resin system upon exposureto actinic radiation; said resin system being further characterized byhaving an absorbance of light in the 330 to 700 nm region of less than0.1 for a 2 mil thick film.
 2. The invention as defined in claim 1wherein there is between about 25 and about 35 percent of the polyolresin between about 20% and about 30% of the epoxidized octafunctionalbisphenol A formaldehyde novolak resin; between about 40 and about 45%of the epoxidized glycidyl ether of tetrabromo bisphenol A resin; andbetween about 2 and 5 parts by weight of the cationic photoinitiator. 3.The invention as defined in claim 1 wherein the molecular weight of thepolyol resin is between about 60,000 and about 90,000; the molecularweight of the epoxidized octafunctional bisphenol A formaldehyde and themolecular weight of the epoxidized glycidyl ether of tetrabromobisphenol A is between about 1,000 and about 1,700.
 4. The invention asdefined in claim 1 wherein an effective amount of up to 10% ofphotosensitizer is included.
 5. The invention as defined in claim 1wherein the photoinitiator is a sulfonium salt.
 6. The invention asdefined in claim 4 wherein the photosensitizer is selected from thegroup anthracene and its derivatives and perylene and its derivatives.7. The invention as defined in claim 1 further characterized by aneffective amount up to about 1 part by weight of a surfactant.
 8. Theinvention as defined in claim 1 further characterized by an effectiveamount up to about 1 part by weight of a dye.
 9. The invention asdefined in claim 1 wherein there is about 20.0% polyol resin about 30.0%of the epoxidized octafunctional bisphenol A formaldehyde novolak resin;about 50.0% of the epoxidized glycidyl ether of Tetrabromo bisphenol Aresin; about 5 parts by weight of sulfonium salt initiator.
 10. Theinvention as defined in claim 1 wherein there is about 27.3% polyolresin about 27.3% of the epoxidized octafunctional bisphenol Aformaldehyde novolak resin; about 45.4% of the epoxidized glycidyl etherof TEtrabromo bisphenol A resin; about 5 parts by weight of sulfoniumsalt initiator.
 11. The invention as defined in claim 1 wherein there isabout 33.3% polyol resin about 25.5% of the epoxidized octafunctionalbisphenol A formaldehyde novolak resin; about 41.7% of the epoxidizedglycidyl ether of Tetrabromo bisphenol A resin; about 5 parts by weightof sulfonium salt initiator.
 12. The invention as defined in claim 1further characterized by said resin being dispersed in a moderatelypolar solvent.
 13. The invention as defined in claim 12 wherein saidsolvent is propylene glycol monomethyl ether acetate.
 14. The inventionas defined in claim 13 wherein there is a concentration of about 40%solids in the solvent.
 15. A photosensitive cationically polymerizableepoxy based imaging system comprising;an epoxy resin system consistingessentially of between about 10% and about 80% by weight of a polyolresin which is a condensation product of epichlorohydrin and bisphenol Ahaving a molecular weight of between about 40,000 and about 130,000;between about 20% and about 90% the weight of an epoxidizedoctafunctional bisphenol A formaldehyde novolak resin having a molecularweight of between about 4,000 and about 10,000; and from about 0.1 toabout 15 parts by weight of a cationic photoinitiator capable ofinitiating polymerization in said epoxidized resin system upon exposureto actinic radiation; said resin system being further characterized byhaving an absorbance of light in the 330 to 700 nm region of less than0.1 for a 2 mil thick film.
 16. The invention as defined in claim 15further characterized by an effective amount up to about 50% by weightof an epoxidized glycidyl ether of tetrabromo bisphenol A resin.
 17. Theinvention as defined in claim 15 wherein the molecular weight of thepolyol resin is between about 60,000 and about 90,000; and the molecularweight of the epoxidized octafunctional bisphenol A formaldehyde novolakresin is between about 5,000 and about 7,000.
 18. The invention asdefined in claim 15 wherein an effective amount of up to 10% ofphotosensitizer is included.
 19. The invention as defined in claim 15wherein the photoinitiator is a sulfonium salt.
 20. The invention asdefined in claim 18 wherein the photosensitizer is selected from thegroup anthracene and its derivatives and perylene and its derivatives.21. The invention as defined in claim 19 wherein there is about 50.0%polyol resin and about 50.0% of the epoxidized octafunctional bisphenolA formaldehyde novolak resin.
 22. The invention as defined in claim 19wherein there is about 73.0% polyol resin and about 27.0% of theepoxidized octafunctional bisphenol A formaldehyde novolak resin. 23.The invention as defined in claim 15 further characterized by said resinbeing in a moderately polar solvent.
 24. The invention as defined inclaim 23 wherein said solvent is propylene glycol monomethyl etheracetate.
 25. The invention as defined in claim 24 wherein there is aconcentration of about 40% solids in the solvent.
 26. The invention asdefined in claim 1 wherein there is about 30.0% of the polyol resin;about 25.0% of the octafunctional bisphenol A formaldehyde novolakresin; about 45.0% of the Tetrabromo bisphenol A resin; and about 5parts by weight of sulfonium salt initiator.